Blow across the top of a bottle until you make a note, remember this sound.

Add some water

Blow across the bottle again. Is the sound the same?

Try different bottles, see what effect this has.

What may Happen

You should find that the pitch of the note gets higher as you add more water.

If you try other bottles you should find that the large ones give a lower note than the small ones.

If you are musical and have you may be able to play a tune.

What is going on?

When you blow on the edge of the bottle you are making a jet of air, if you hold your fingers nearby you can feel it. If you are making a noise this jet of air could either be deflected into the bottle or over the top of it, and it only takes a small force to change between these two paths.

If the air starts off going into the bottle, there is nowhere for it to escape so the pressure builds up, at some point the pressure builds up so much that the jet is pushed out of the bottle. Now the fast moving air sucks air out of the bottle until the pressure is low enough to suck the jet back in. You have made a vibration.

Air going into the bottle

The air flows over the top

The air stream is sucked into the bottle, which pushes extra air into the bottle

Eventually the pressure inside the bottle gets high enough to push the jet outside again. Now the fast moving air of the jet sucks air out of bottle, reducing the pressure inside.

Why does adding water make the pitch change?

Pitch is just a measure of how fast the vibration is that reaches your ear, rapid vibrations mean high pitches. In a large bottle there is a lot of space so it takes more air flowing in to build up enough pressure to push the jet out again. Getting more air into the bottle takes a relatively long time so the air vibrates slowly (at maybe 100 vibrations a second).

If you add water or use a smaller bottle there is less space available so it takes less time for the pressure to build up. This means the vibration happens more quickly and so produces a higher pitch.

Empty Bottle

Half full bottle

An empty bottle has a large volume which means that the air behaves like a weak spring, because a lot of air has to move in to change the pressure much.

A half full bottle of water has less space for air inside, so it behaves as a stiffer spring, so it will vibrate faster.

This is the principle that a flute works on, but instead of adding water you change the effective length of the tube by opening and closing holes.

Squashing Bottles

What happens to the note a bottle makes when you squash it? What would happen if you then put it underwater? Find out in this kitchen science.

What you need

A bottle

A plastic bottle, 500ml smooth sided fizzy drinks bottles work well

A jug

A container nearly as deep as your bottle is tall

What to Do

Blow across the bottle, remember the note it makes.

Squash the bottle

Blow across it again. Is there a difference in the note?

Fill your container about 3/4 full of water

See if immersing the bottle with water affects the note it makes, when it is both squashed and unsquashed.

What may Happen

You should find that squashing the bottle makes the note go down.

When the bottle is immersed it will increase the pitch of the squashed bottle, but not the unsquashed one.

What is going on?

Vibration

When you blow over the top of the bottle the stream of air you produce can either be deflected into or out of the bottle. If it starts off going into the bottle the pressure will build up until the stream is pushed out, and then as the bottle empties the pressure drops and the stream can move back in. This produces a vibration you hear as sound. As you can find from the musical bottles experiment, if the bottle is large it takes a long time for the pressure to build up so the vibration is slow and you hear a low note.

If a plastic bottle is flattened the changes in pressure cause the bottle can change shape slightly, if you touch the flattened area while you are blowing you can feel this as a strong vibration. This change in shape allows more air to flow in for the same change in pressure, so the speed of the vibration and therefore the pitch is lower than it should be for the amount of air available.

If you surround the bottle with water by immersing it, the water effectively stops the walls from moving. This means that the pitch is correct for the amount of air in the bottle - which is of course reduced from it's unsquashed volume so the pitch is higher than normal.

Flat Bottle

Flat bottle in water

Cross section through a squashed bottle. When the pressure rises or falls it will push outwards or inwards making the bottle more or less circular.

The dense water stops the sides moving.

If the bottle is unsquashed on the other hand, because it is circular it can't get any larger without more plastic, and because there are no weak spots it is hard for it to get smaller. This means that the walls do not vibrate in the same way as in the squashed bottle, and putting the bottle in water will make no difference to the sound.

A round bottle

A round bottle in water

In air a round bottle can't change shape if the pressure increases or decreases

So when you move it into water it still can't change shape so nothing has changed